Vapor deposition processes are used to deposit compounds in a gas phase onto the surface of a solid substrate. Vapor deposition processes are widely used, and can generally be classified into two groups: chemical and physical. In chemical vapor deposition (CVD), reactants in the gas phase flow over a heated substrate and react at or near the surface of the substrate, thereby depositing a film. These carrier gases are commonly employed to dilute the gaseous reactants used in CVD, many of which are toxic or otherwise hazardous.
CVD is used in the deposition of both the polycrystalline and epitaxial silicon layers upon a silicon wafer. Silane, silicon tetrachloride, trichlorosilane, and/or dichlorosilane gases are typical gas phase reactants used to produce these silicon layers. In particular, it is known that a layer of polycrystalline silicon may be deposited from pure silane in the gas phase using low pressure CVD. Typically, the layer of polycrystalline silicon is deposited upon the back side of the wafer to serve a gettering function. In addition, silane gas may also be used in the atmospheric pressure CVD of silicon dioxide upon a silicon wafer from a gaseous mixture containing silane, thereby effectively sealing the back side of the wafer and significantly reducing autodoping.
Excess gas phase reactant remains in the CVD system subsequent to its reaction with the substrate, as is common in many types of chemical reactions. This excess reactant, typically a mixture of one of more silicon compounds in a carrier gas, is removed from the deposition chamber via an exhaust system. In one particular example, an effluent stream produced by the CVD of elemental silicon contains silicon hydrides, namely silanes and disilanes, as well as finely dispersed particulate silicon.
CVD exhaust systems are typically designed to vent treated effluent process streams to the atmosphere. Therefore, the presence of either silane or disilane gases in the CVD effluent stream is especially problematic, as both of these gases are pyrophoric. Due to their hazardous properties, silanes and disilanes can not be released directly into the atmosphere. Therefore, any silanes and disilanes remaining in the exhaust gas must be reacted in order to remove them from the CVD effluent stream prior to its emission to the air. At present, several techniques exist to remove gaseous hydrides such as silane from a gaseous mixture. These techniques include processes by which silane is either filtered from the gas stream or converted into compounds which are suitable for disposal in the environment. Typical methods include the use of semi-permeable membranes, thermal methods such as pyrolysis and oxidation, and wet scrubbing. Currently, each CVD line typically has a dedicated exhaust system for processing effluent gases using a combination of such methods.
Semi-permeable membranes are available to filter silane gas from a mixture of gases. Such membranes are primarily used to purify the silane gas for reuse in the deposition reaction. Semi-permeable membranes are discussed in U.S. Pat. Nos. 5,503,657; 5,131,927 and 4,941,893 and are well known in the art.
Thermal pyrolysis converts silane into polycrystalline silicon and hydrogen gas by treating the CVD effluent gas in a furnace. Furnaces are likewise known for use in thermal oxidation used to produce silicon dioxide from silane. However, neither thermal pyrolysis or oxidation is capable of completely converting all the silane contained in the effluent stream. Therefore, a second separation apparatus must be used to remove residual silane from the furnace's effluent stream prior to its emission to the atmosphere. One method for removing the residual silane is to convert the silane into silicon dioxide, either by a secondary thermal oxidation process or wet scrubbing. The silicon dioxide, a chemically inert material, may then readily be disposed of in the environment.
Thermal oxidation involves the combustion of a gaseous mixture containing silane. A common secondary thermal oxidation technique is the use of "burn boxes," in which a combustible gas is introduced into the gaseous stream and ignited. Alternatively, if the carrier gas employed is flammable, such as hydrogen, the effluent from the primary separation apparatus may simply be flared following initial separation. However, as with all combustion reactions, these methods generate unwanted by products, such as NO.sub.x and CO.sub.2.
In wet scrubbing, a chemical reaction is induced between the residual silane and water, thereby forming silicon dioxide. Wet scrubbing is advantageous since wet scrubbing does not produce the harmful byproducts generated during combustion reactions. Wet scrubber systems are well known in the art for removing materials from a gaseous stream. In general, wet scrubber systems remove deleterious materials from gaseous mixtures by bringing the gas mixture into intimate contact with a scrubbing liquid. The scrubbing liquid is chosen so as to dissolve or react with the deleterious component, thus removing it from the gas mixture. Following intimate contact, the scrubbed gas is allowed to escape from the spent scrubbing liquid. Subsequently, the scrubbed gas is filtered to remove any particulates entrained in the gas stream. Several means are available by which to provide contact between the liquid and the gas, including jet pumps and spray nozzles.
Wet scrubbers are known in the art to remove residual silane from an effluent gas stream. Further, it is known to contact a gaseous mixture containing silane with water in a wet scrubber by means of a jet pump. Such wet scrubber units consist of a jet pump attached to the top of a housing in which the gas is allowed to separate from the water. Spent water collects in the bottom portion of the housing, while the scrubbed gas mixture fills the headspace above the spent water. A gas outlet port at the top of the housing allows the scrubbed gas to eventually escape to the atmosphere.
The silicon dioxide produced during the wet scrubbing process is in the form of fine particulates, a portion of which is entrained by the scrubbed gas mixture during the separation process, the remainder of which remains in solution in the water. The silicon dioxide may be removed from the spent water by simply allowing it to settle out of solution, or any other method known in the art, and thereafter disposed of safely as an inert solid waste. However, although chemically inert, the fine particulates entrained in the scrubbed gas may present an inhalation hazard. Therefore, the scrubbed gas mixture is generally filtered prior to its release to the atmosphere to avoid emission of these fine particulates into the atmosphere. At present, this filtration step introduces severe manufacturing inefficiencies into the entire CVD process.
Currently, scrubbed gas is passed through filtration media which has been inserted into the gas outlet port at the top of the separation housing. This filtration media consists of layers of wire mesh, cut to equal size, which have been stacked one on top of another in parallel. A stack of such layers of filter media is held together by metal rods which run perpendicular to the individual layers through pre-cut holes provided in each filter, thus forming a filter pack. This filter pack is secured to the top of the separation housing by bolts or screws. Once in place, the filter pack occupies the entire gas outlet port, extending vertically down into the headspace of the separation housing. Typically, the filter pack is quite thick, such as about 18 to 20 inches in most instances.
To change out filter packs when they become clogged, the entire wet scrubber unit must be shut down to prevent particulates from entering the airstream during the changeover process. More importantly, when the wet scrubber unit is shut down, the CVD line to which the wet scrubber unit is dedicated must be shut down, as well. Changing spent filtration media is further a time consuming process. To change out filter packs, the spent filter pack is loosened from the top of the separation housing. The spent filter pack is then lifted vertically out of the gas outlet port, which remains uncovered until such a time as a fresh filter pack is inserted. Only after securing the fresh filter pack into place may the CVD line and dedicated scrubber resume operation. Filters are typically changed on a monthly basis, and the procedure can be quite lengthy. This down time yields significant production losses. As a result of this loss of production incurred during filter change, each CVD line typically has an individual scrubber dedicated to it, thereby requiring only a single CVD line to be shut down while the filter is being changed but significantly increasing capital costs.